This invention relates generally to a method and apparatus for transporting a web in a paper making machine to a flow-through dryer and, more particularly, to a method and apparatus for transferring a web from a pick-up fabric and transporting it to a flow-through dryer by means of a flow-through drying wire.
The use of flow-through type dryers in paper machines and, especially in connection with tissue paper machines, has recently become more extensive. Such flow-through type dryers generally include a drying cylinder having a perforated mantle and a surrounding hood having a pressurized interior. In such flow-through dryers, hot drying gases are directed through the web and cylinder mantle to the low pressure or vacuumed interior of the flow-through drying cylinder or vice-versa.
The use of such flow-through drying apparatus is common in tissue machines which do not use a wet press so that the softness, bulky nature and absorbing capacity of the tissue paper is maintained.
Tissue paper making machines are known which include large diameter flow-through drying cylinders which direct a web to a subsequent creping cylinder from which the web is removed in a known fashion by creping scrapers. Tissue paper machines are also known utilizing conventional yankee cylinders and subsequent flow-through dryer cylinders which function as afterdryers. For example, reference is made to the article "Fortschritte in der Durchstromtrockungstechnik" in "Das Papier", No. 10a 1976, pages 118 to 127.
Previously, tissue webs have been transferred to flow-through dryer cylinders by means of blower apparatus which direct a blowing effect on the web through the wire which carries it. More particularly, web transfer has been accomplished by providing a suction box within the interior of the flow-through dryer cylinder, usually on the axle thereof, for applying suction on the web supporting wire engaged on the drying cylinder which tends to remove the web from the carrying wire securing it to the mantle of the flow-through dryer cylinder.
The previous methods and apparatus for transferring the tissue web to the flow-through dryer are not entirely satisfactory. Thus, the use of such prior methods and apparatus have been limited to cases where the web carrying fabric comprises a wire structure. Thus, the use of carrying fabrics formed of felt which would otherwise be desirable to facilitate water removal without wet pressing has heretofore not been possible. Further, the provision of suction boxes within the interior of the flow-through dryer cylinders or similar arrangements are relatively complicated and expensive in manufacture since, among other reaons, these devices must be supported on the axle of the flow-through dryer cylinder necessitating relatively complicated construction.
Reference is made to U.S. Pat. Nos. 3,303,576 and 3,821,068 relating generally to the subject matter of the present invention. Reference is also made to copending U.S. Application Ser. No. 922,275, filed July 6, 1978, assigned to the assignee of the present application, which discloses related subject matter.